This invention relates to infrared light modulators, and more particularly to infrared liquid crystal light modulators, including reflective and transmissive ferroelectric liquid crystal infrared xe2x80x9cchoppers.xe2x80x9d
Like visible light, infrared light is a form of electromagnetic radiation. It occupies the portion of the electromagnetic wavelength spectrum between 750 nm and 1000 xcexcm. However, unlike visible light, infrared radiation is not visible to humans. Nevertheless, useful scientific information may be obtained by observing with specialized instruments the infrared radiation emitted from or reflected by objects from atoms to stars. Although the term xe2x80x9clightxe2x80x9d is often used to refer only to visible electromagnetic radiation, for ease of discussion in this application the term infrared light will be used along with infrared radiation, to refer to electromagnetic radiation in the infrared region.
One form of infrared measurement, known as radiometry, measures the intensity of infrared light a source emits, absorbs, or reflects by comparing the observed infrared light to a reference measurement. Radiometry is useful, for instance, in analyzing the chemical constituents in a gas sample. This subclass of radiometry is known as spectro-radiometry.
Spectro-radiometry makes use of the fact that all elements or compositions of matter have an infrared xe2x80x9csignaturexe2x80x9d or xe2x80x9cfingerprintxe2x80x9d. Matter absorbs or reflects infrared radiation to some degree, depending on the wavelength of the radiation. By observing the pattern of infrared light reflected from or absorbed by a sample of unknown composition, it becomes possible to determine what elements and/or compounds are present in the sample. A spectro-radiometer used for such investigation directs various wavelengths of infrared light at the sample and observes to what degree the radiation is absorbed at each wavelength. Alternatively, a spectro-radiometer may direct infrared light containing a number of different wavelengths at the sample and observe the infrared light after passing through the sample using a number of infrared detectors, each configured to observe specific wavelengths.
Radiometers, like other types of infrared detectors that compare reflected infrared light to a reference measurement, generally require an alternating infrared source. Infrared light from the source is alternately directed at the detector through the sample and blocked from passing to the detector. The detector then compares the difference between the two measurements to produce an output signal representative of the difference. In order to increase the accuracy of such systems, it is important to have a consistent, and stable source of infrared light. That is, the alternating infrared waveform should have equal intensity and spectral composition from one ON pulse to the next, each OFF pulse should be equally dark, and the transitions between ON and OFF pulses should be consistent.
One method for providing an alternating infrared light source is to flash the actual source. However, the internal heat generated by infrared sources along with other factors limit the source""s ability to emit infrared light of constant intensity or wavelength throughout a pulse when the source is pulsed ON and OFF. Therefore, this is an unreliable method of providing stable, alternating infrared light for many applications.
Another method for providing an alternating infrared source is through the use of mechanical xe2x80x9cchoppersxe2x80x9d, such as shutters or rotating wheels with apertures to alternately block and pass the light. Mechanical choppers work well for many applications. However, many infrared sensor applications require very small choppers, for which mechanical choppers are not very well suited. Further, the moving parts in mechanical choppers may introduce unwanted vibration into the detection system, thus reducing the detector""s usefulness. Additionally, components with moving parts are inherently less reliable than those with no moving parts. Finally, mechanical choppers are more difficult to control than similar electromechanical devices, making it more difficult to control the speed and accuracy with which they operate.
The use of mechanical choppers for visible light results in many of the same limitations and disadvantages. For that reason, it is well known to use liquid crystals for visible light choppers. However, liquid crystals have a number of limitations that make them potentially unsuitable for use in infrared choppers, and to applicants"" knowledge, liquid crystals have not been used in infrared choppers. Liquid crystal chopper components behave differently in infrared wavelengths in ways that make them potentially unsuitable for liquid crystal infrared choppers. First, inexpensive polarizers used in visible light applications are not effective at infrared wavelengths, and alternatives are expensive. Further, liquid crystals have high absorption of light in infrared wavelengths, thus substantially reducing the amount of infrared light available. Finally, the window material used to contain the liquid crystal must be something other than glass, which also absorbs infrared light, and alternatives to glass are also expensive.
Therefore, the need exists for an infrared chopper that is small, reliable, and as free of moving parts as possible. If this is to be done with liquid crystals, there are many technical challenges to be solved before a chopper can be produced that is sufficiently economical to be used in various systems. It is against this background and a desire to solve the problems of the prior art that the present invention has been developed.
The present invention relates generally to a liquid crystal infrared light modulator that can be driven by electrical signals to modulate incoming infrared light. The modulator includes a polarizer receptive of the incoming light that produces polarized light and a layer of liquid crystal material switchable between at least two states, the liquid crystal layer acting on polarized IR light from the polarizer to provide a first polarization state of polarized light if the liquid crystal is in a first state and to provide a second polarization state of polarized light if the liquid crystal is in a second state. It also includes a pair of IR transparent, conductive substrates positioned on either side of the liquid crystal layer that are suitable for having voltages applied thereto to drive the liquid crystal layer to one of the two states. It further includes an analyzer that substantially blocks polarized light of the first polarization state when the liquid crystal layer is in the first state and substantially passes polarized light of the second polarization state when the liquid crystal layer is in the second state.
The liquid crystal material may be ferroelectric. The ferroelectric liquid crystal material may have a birefringence greater than 0.17 at a wavelength of 589 nm. The ferroelectric liquid crystal material may have an average transmissivity across the electromagnetic spectrum from 8-13 xcexcm greater than 50%. At least one of the conductive substrates may include Germanium. The Germanium may be doped to increase the conductivity thereof. At least one of the polarizer and the analyzer may be constructed by a lithographic process. The polarizer and analyzer may be in a crossed-polarizer configuration relative to one another.
The present invention is also related to a liquid crystal infrared light modulator that can be driven by electrical signals to modulate incoming infrared light. The modulator includes a polarizer receptive of the incoming light that produces polarized light and a layer of liquid crystal material switchable between at least two states, the liquid crystal layer acting on polarized IR light from the polarizer to provide a first polarization state of polarized light if the liquid crystal is in a first state and to provide a second polarization state of polarized light if the liquid crystal is in a second state. It also includes an analyzer that substantially blocks polarized light of the first polarization state when the liquid crystal layer is in the first state and substantially passes polarized light of the second polarization state when the liquid crystal layer is in the second state. It further includes a first conductive substrate positioned on one side of the liquid crystal layer, the first substrate having a reflector thereon that reflects IR light such that incoming polarized light passes twice through the liquid crystal layer before passing through the analyzer and a second conductive substrate transparent to IR light positioned on the opposite side of the liquid crystal layer from the first substrate, the first and second substrates cooperating to modulate the liquid crystal layer between the two states.
The present invention also relates to a liquid crystal infrared light modulator that modulates incoming polarized IR light such that the IR light is either substantially blocked or passed by an analyzer after traveling through the modulator. The modulator includes a pair of conductive substrates and a layer of liquid crystal material positioned between the substrates, the liquid crystal layer operating between at least two states, a first state that provides a first polarization state of polarized light from the incoming polarized IR light and a second state that provides a second polarization state of polarized light from the incoming polarized IR light. The conductive substrates can be driven by electrical signals to modulate the liquid crystal between the two states, the substrates and the liquid crystal layer configured in either of two configurations, a transmissive configuration, wherein the IR light passes through the liquid crystal layer only once, and a reflective configuration, wherein the IR light passes through the liquid crystal layer twice.
The present invention also relates to an infrared detection system that analyzes the characteristics of a composition of matter. The detection system includes a source of infrared light and an IR chopper that alternately substantially passes and substantially blocks light from the IR light source. The chopper includes a polarizer receptive of incoming infrared light that produces polarized light, a layer of liquid crystal material switchable between at least two states, the liquid crystal layer acting on polarized IR light from the polarizer to provide a first polarization state of polarized light if the liquid crystal is in a first state and to provide a second polarization state of polarized light if the liquid crystal is in a second state, a pair of IR transparent, conductive substrates positioned on either side of the liquid crystal layer for modulating the liquid crystal layer between the two states, and an analyzer that substantially blocks polarized light of the first polarization state when the liquid crystal layer is in the first state and substantially passes polarized light of the second polarization state when the liquid crystal layer is in the second state. The detection system also includes an IR detector that senses the IR light after the IR light passes through the composition of matter. The composition of matter is located such that the IR light passes therethrough either before or after the IR light passes through the liquid crystal IR chopper.
The present invention also relates to a liquid crystal infrared light modulator that can be driven by electrical signals to modulate incoming infrared light. The modulator includes a reflective polarizer receptive of the incoming light that produces polarized light and a liquid crystal cell. The cell includes a layer of liquid crystal material switchable between at least two states, the liquid crystal layer acting on polarized IR light from the polarizer to provide a first polarization state of polarized light if the liquid crystal is in a first state and to provide a second polarization state of polarized light if the liquid crystal is in a second state. The cell also includes a first conductive substrate positioned on one side of the liquid crystal layer, the first substrate having a reflector thereon that reflects IR light such that incoming polarized light passes twice through the liquid crystal layer before passing through the analyzer, and a second conductive substrate transparent to IR light positioned on the opposite side of the liquid crystal layer from the first substrate, the first and second substrates cooperating to modulate the liquid crystal layer between the two states. The modulator also includes a reflective analyzer that substantially blocks polarized light of the first polarization state when the liquid crystal layer is in the first state and substantially passes polarized light of the second polarization state when the liquid crystal layer is in the second state. At least two of the polarizer, the liquid crystal cell, and analyzer are positioned in a substantially coplanar relationship with one another.